A base station for a cellular telephone system uses expensive, environmentally sensitive electronics. Typically mounted outdoors, base stations are exposed to extreme weather conditions, including harshly cold or warm temperatures. A weather resistant enclosure is often used to protect the base station electronics from the environment. It is known to provide an outer cover or solar shield to such an enclosure, which is beneficial in sheltering the enclosure from solar heat. A solar shield, however, can impede the dissipation of heat from the base station.
A base station also requires a constant source of electrical power to maintain operation. Since electric power outages are undesired, back-up power supplies, like lead acid batteries, are used to ensure continuous power to the electronics. Should the primary power supply fail, the battery powers the electronics for a given time. A lead acid battery, however, only operates in a limited temperature range. If a battery falls below 20 degrees centigrade, its output is drastically reduced. Similarly, a battery exposed to extreme warmth will have its useful life drastically shortened. It is therefore desirable to maintain battery temperature below 65 degrees centigrade.
To protect base station electronics and batteries from high temperatures, base stations are provided with heat dissipating structures. It is known to provide a heat sink, including thermally conductive fins, on the base station enclosure and under the solar shield for dissipating heat generated within the enclosure. The fins typically define vertical channels which act as chimneys, allowing heated air to rise and escape through an opening at the top of the solar shield. As the enclosure is cooled, so is the battery.
To protect against battery failure due to low temperature, base stations manufactured for use in cold environments are often equipped with in electric heater dedicated to the battery. It is known in the art to mount the battery in direct physical contact with the heater and to enclose the battery, battery heater and base station electronics in the base station enclosure. If the temperature within the enclosure falls below a certain threshold, electric power is delivered to the heater, thereby heating the battery.
There are several disadvantages in using a dedicated electric heater to heat a battery within a base station enclosure. Electric heaters are expensive and occupy space within the enclosure, increasing the cost of the assembly. Since the heater must be powered, system design must account for the power drawn when the heater is active. For instance, base stations using an electric heater for the battery must be equipped with components, such as circuit breakers, capable of handling increased loads to accommodate the current drawn by the battery heater. Batteries in contact with electric heaters also place the electronics at risk. If an electric heater in direct contact with a battery short circuits, or if the heater control fails, the battery will overheat. It is known for an overheated battery to leak acid. In a cellular base station, an acid leak may damage or destroy expensive electronics, create a fire hazard or cause the release of dangerous gases.
Eliminating the use of an electric heater to heat a battery in a cellular system base station is highly desirable. Removing the heater and associated structures from the base station enclosure conserves space. The risks associated with loss of heater control, such as an acid leak, are substantially eliminated. Power system components, like circuit breakers, need not account for battery heater loads. There is a need, therefore, for a battery enclosure which will maintain the operating temperature of a lead acid battery without use of a dedicated electric heater.